The wires may just be to light in this case regardless of what the recommendations are, the mistake was made before. If there is a chafed wire in the circuit upstream somewhere the conductor leading back to the 50 amp fuse and its holder needs to be able to carry the current to blow the fuse. On a short circuit the conductor most likely would carry the current and blow the fuse, but on leakage current such as a chafed wire it stays below the threshold of blowing the fuse and heats the conductor and its circuit components. This heating reduces the current so the fuse will not drop, the cycle continues, more heat, less current until a meltdown occurs. A large enough wire would be able to carry the same current for an indefinite period of time without the heating until it becomes an overload to blow the fuse.
This seems to be a common scenario with chassis wiring, fuses are more for short circuit protection rather than overload protection. Thank your stars that is not the way your house is wired.